1. Field of the Invention
The present invention relates to a slip control system for a motor vehicle and, in particular, to a slip control system which includes a traction control system and an antiskid brake system.
2. Description of Related Art
There have been proposed slip control systems with a traction control system for a motor vehicle, Such is shown in Japanese Laid-Open Patent Publication No. 58-16948, No. 57-22948, No. 62-231836, and the like. When an excessive slip is caused between driving wheels and road surfaces during starting and/or running operations, the slip control system decreases the excessive slip to under a predetermined value to obtain driving forces which are transmitted to the road surface from the driving wheels. Therefore, starting or accelerating ability is improved.
When the vehicle is running on a low friction coefficient road surface, an excessive depression of the acceleration pedal causes an excessive slip in the driving wheels. At this time, the traction control system decreases the driving torque transmitted to the driving wheels by increasing the brake fluid pressure in the driving wheels and increasing the brake force on the driving wheels. As a result, the vehicle decreases the excessive slip and obtains a desired friction coefficient between the driving wheels and the road surface.
The slip control system is provided with an antiskid brake system (hereinafter called "ABS") which maintains a desired friction coefficient between the driving wheels and the road surface and obtains an optimal brake force in the braking operation by controlling the brake fluid pressure in the driving wheels based on the skid condition in the wheels. The ABS detects the skid condition of the respective wheels based on the estimated vehicle speed computed by using the reduction speed of the vehicle and the wheel rotation speed of the respective wheel and then, based on the skid condition, decreases the brake fluid pressure so as to cancel the lock condition of the wheels in the braking condition. Consequently, the vehicle obtains a desired friction coefficient between the driving wheels and the road surface.
In a conventional slip control system including both the traction control system and the ABS, for the safety of passengers, brake control by the ABS is given priority over the brake control by the traction control system. There is a possibility that the ABS will mistake the reduction speed of the driving wheel caused when the traction control system is controlling the slip of the driving wheel for the reduction speed of the wheel caused by the skid condition of the wheel. Consequently, the brake fluid pressure which should be maintained in the increased pressure condition is, to the contrary, decreased.
Moreover, the ABS employs a right and left independent control in the front wheels by which the ABS controls the brake fluid pressure in the right and left front wheels independently. The ABS also employs a right and left integrated control in the rear wheels to stabilize the vehicle, by which the ABS controls the brake fluid pressure in the right and left rear wheels, based on the reduction speed of a rear wheel which contacts a road surface having a friction coefficient which is relatively small as compared to that of another road surface or the average of the reduction speed of the rear wheels.
On the other hand, the traction control system employs a right and left independent control in the driving wheels by which the system controls the brake fluid pressure in the right and left driving wheels independently so as to maintain the driving torque in the driving wheel with no slip and improve drivability.
Therefore, when the brake fluid pressure in the rear or driving wheels starts to be decreased by the ABS immediately after the traction control system increases the brake fluid pressure in the rear or driving wheels, the ABS decreases the brake fluid pressure in the right and left rear wheels equally based on the rear wheel contacting the road surface having a friction coefficient which is relatively small as compared to that of another road surface or the rear wheel having a brake fluid pressure which is increased higher than that of the other rear wheel by the traction control system. Consequently, there is a possibility that the brake fluid pressure in the rear wheel having a brake fluid pressure which is not increased higher than that of the other rear wheel will be increased too much.